Method for plating a moving metal strip and coated metal strip produced thereby

ABSTRACT

A method for producing a steel substrate coated with a chromium metal-chromium oxide (Cr—CrOx) coating layer in a continuous high speed plating line, operating at a line speed (v1) of at least 100 m·min −1 , wherein one or both sides of the electrically conductive substrate in the form of a strip, moving through the line, is coated with a chromium metal-chromium oxide (Cr—CrOx) coating layer from a single electrolyte by using a plating process. A coated steel substrate and a packaging made thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a § 371 National Stage Application of International ApplicationNo. PCT/EP2015/061332 filed on May 21, 2015, claiming the priority ofEuropean Patent Application No. 14169312.7 filed on May 21, 2014.

This invention relates to a method for producing a coated steelsubstrate in a continuous high speed plating line and to a coated metalstrip produced using said method.

Electroplating or (in short) plating is a process that uses electricalcurrent to reduce dissolved metal cations so that they form a coherentmetal coating on an electrode. Electroplating or electrodeposition isprimarily used to change the surface properties of an object (e.g.abrasion and wear resistance, corrosion protection, lubricity, aestheticqualities, etc.). The part to be plated is the cathode in the circuit.Usually, the anode is made of the metal to be plated on the part. Bothcomponents are immersed in a solution called an electrolyte containingone or more dissolved metal salts as well as other ions that permit theflow of electricity. A power supply supplies a direct current to theanode, oxidizing the metal atoms that comprise it and allowing them todissolve in the solution. At the cathode, the dissolved metal ions inthe electrolyte solution are reduced at the interface between thesolution and the cathode, such that they “plate out” onto the cathode.The rate at which the anode is dissolved is equal to the rate at whichthe cathode is plated, vis-a-vis the current flowing through thecircuit. In this manner, the ions in the electrolyte bath arecontinuously replenished by the anode.

Other electroplating processes may use a non-consumable anode such aslead or carbon. In these techniques, ions of the metal to be plated mustbe replenished in the bath as they are drawn out of the solution.

Chromium plating is a technique of electroplating a thin layer ofchromium onto a metal object. The chromium layer can be decorative,provide corrosion resistance, or increase surface hardness.

Traditionally, the electrodeposition of chromium was achieved by passingan electrical current through an electrolyte solution containinghexavalent chromium (Cr(VI)). However, the use of Cr(VI) electrolytesolutions is problematic in view of the toxic and carcinogenic nature ofCr(VI) compounds. Research in recent years has therefore focused onfinding suitable alternatives to Cr(VI) based electrolytes. Onealternative is to provide a trivalent chromium Cr(III) based electrolytesince such electrolytes are not toxic and afford chromium coatingssimilar to those that are deposited from Cr(VI) electrolyte solutions.

For some types of packaging steels chromium coated steel is produced.Chromium coated steel for packaging purposes is normally a sheet orstrip of steel electrolytically coated with a layer of chromium andchromium oxide with a coating thickness of <20 nm. Originally called TFS(Tin Free Steel), it is now better known by the acronym ECCS(Electrolytic Chromium Coated Steel). ECCS is typically used in theproduction of DRD (Drawn & Redrawn) two-piece cans and components thatdo not have to be welded, such as ends, lids, crown corks, twist-offcaps and aerosol bottoms and tops. ECCS excels in adhesion to organiccoatings, both lacquers and polymer coatings, like PET or PP coatings,which provide robust protection against a wide range of aggressivefilling products, as well as excellent food safety standards, being bothBisphenol A and BADGE free. Up till now ECCS was produced based on aCr(VI) process. Conventional Cr(III) processes proved to be incapable ofreplicating the quality of the Cr(VI) based layers because the Cr(III)processes resulted in amorphous and/or porous layers, rather thancrystalline and dense layers. However, recent developments show thatcoating layers can be successfully deposited on the basis of aCr(III)-based electrolyte as demonstrated by WO2013143928.

In industrial processes it is important to produce quickly and costeffectively. However, conventional processes result in the need to applyincreasing current densities with increasing strip speeds. Highercurrent densities result in a faster deposition rate, but also in highercosts for electricity and for high electric power equipment.

It is an object of the present invention to provide a method thatprovides a chromium-chromium oxide (Cr—CrOx)layer on a steel substratein a single plating step at high speed with lower plating currentdensities.

It is also an object of the present invention to produce achromium-chromium oxide (Cr—CrOx) layer on a steel substrate in a singleplating step at high speed from a simple electrolyte.

It is also an object of the present invention to produce achromium-chromium oxide (Cr—CrOx) layer by plating it on a steelsubstrate at high speed from a simple electrolyte based on trivalent Crchemistry.

One or more of these objects can be achieved by for producing a steelsubstrate coated with a chromium metal-chromium oxide (Cr—CrOx) coatinglayer in a continuous high speed plating line, operating at a line speed(v1) of at least 100 m·min⁻¹, wherein one or both sides of theelectrically conductive substrate in the form of a strip, moving throughthe line, is coated with a chromium metal-chromium oxide (Cr—CrOx)coating layer from a single electrolyte by using a plating process,wherein the substrate is a steel substrate which acts as a cathode andwherein the CrOx deposition is driven by the increase of the pH at thesubstrate/electrolyte interface (i.e. surface pH) due to the reductionof H⁺ to H₂(g), and wherein the increase of pH is counteracted by adiffusion flux of H⁺-ions from the bulk of the electrolyte to thesubstrate/electrolyte interface and wherein this diffusion flux ofH⁺-ions from the bulk of the electrolyte to the substrate/electrolyteinterface is reduced by increasing the kinematic viscosity of theelectrolyte and/or by moving the strip and the electrolyte through theplating line in concurrent flow wherein the steel strip is transportedthrough the plating line with a velocity (v1) and wherein theelectrolyte is transported through the strip plating line with avelocity of v2, thereby reducing the current density to deposit CrOx andreducing the amount of H₂(g) formed at the substrate/electrolyteinterface. Dependent on the type of metal, it is possible that some ofthe metal oxide is further reduced to metal. It was found by the presentinventors that this happens in case of Cr.

The term metal oxide encompasses all compounds including Me_(x)O_(y)compounds, where x and y may be integers or real numbers, but alsocompounds like hydroxide Me_(x)(OH)_(y) or mixtures thereof, whereMe=Cr.

A high speed continuous plating line is defined as a plating linethrough which the substrate to be plated, usually in the form of astrip, is moved at a speed of at least 100 m·min⁻¹. A coil of steelstrip is positioned at the entry end of the plating line with its eyeextending in a horizontal plane. The leading end of the coiled strip isthen uncoiled and welded to the tail end of a strip already beingprocessed. Upon exiting the line the coils are separated again andcoiled, or cut to a different length and (usually) coiled. Theelectrodeposition process can thus continue without interruption, andthe use of strip accumulators prevents the need for speeding down duringwelding. It is preferable to use deposition processes which allow evenhigher speeds. So the method according to the invention preferablyallows producing a coated steel substrate in a continuous high speedplating line, operating at a line speed of at least 200 m·min⁻¹, morepreferably of at least 300 m·min⁻¹ and even more preferably of at least500 m·min⁻¹. Although there is no limitation to the maximum speed, it isclear that control of the deposition process, the prevention of drag-outand of the plating parameters and the limitations thereof becomes moredifficult the higher the speed. So as a suitable maximum the maximumspeed is limited at 900 m·min⁻¹.

This invention relates to the deposition of chromium and chromium oxidelayer (Cr—CrOx) from an aqueous electrolyte by means of electrolysis ina strip plating line. The deposition of CrOx is driven by the increaseof the surface pH due to the reduction of H⁺ (more formally: H₃O⁺) toH₂(g) at the strip surface (being the cathode), and not by the regularplating process in which metal ions are discharged by means of anelectrical current according to: Me^(n+)(aq)+n·e⁻→Me(s). In such aprocess, increasing the current density is sufficient to achieve thesame plated thickness when the strip speed increases (provided thediffusion of metal ions to the substrate is not a limiting factor).

In an embodiment this invention relates to the deposition of a chromiumand chromium oxide layer (Cr—CrOx) from a trivalent chromium electrolyteby means of electrolysis in a strip plating line. The deposition of CrOxis driven by the increase of the surface pH due to the reduction of H⁺,and not by the regular plating process in which metal ions aredischarged by means of an electrical current. The linear relationshipshown in FIG. 3 provides evidence for the hypothesis that the depositionof Cr(HCOO)(H₂O)₃(OH)₂(s) on the electrode surface is driven by thediffusion flux. In a second stage, the Cr(HCOO)(H₂O)₃(OH)₂(s) deposit ispartly further reduced to Cr-metal and partly converted into Cr-carbide.

The mechanism of a deposition process from a Cr(III)-based electrolyteis believed to be as follows. When the current density is increased, thesurface pH becomes more alkaline and Cr(OH)₃ is deposited if pH>5. Thisexperimental behaviour can be explained qualitatively by assuming thefollowing chain of equilibrium reactions:Cr³⁺+OH⁻

Cr(OH)²⁺Cr(OH)²⁺+OH⁻

Cr(OH)₂ ⁺Cr(OH)₂ ⁺+OH⁻

Cr(OH)₃

Or, more accurately in case the formate ion (HCOO⁻) is the complexingagent:[Cr(HCOO)(H₂O)₅]²⁺+OH⁻→[Cr(HCOO)(OH)(H₂O)₄]⁺+H₂O  (regime I)[Cr(HCOO)(OH)(H₂O)₄]⁺+OH⁻→Cr(HCOO)(OH)₂(H₂O)₃+H₂O  (regime II)Cr(HCOO)(OH)₂(H₂O)₃+OH⁻→[Cr(HCOO)(OH)₃(H₂O)₂]⁻+H₂O  (regime III)

The regimes I-III are visible when the deposition of chromium is plottedagainst the current density (cf. for example FIG. 4). Regime I is theregion where there is a current, but no deposition yet. The surface pHis insufficient for chromium deposition. Regime II is when thedeposition starts and increases linearly with the current density untilit peaks and drops of in regime III where the deposit starts todissolve.

When the surface pH becomes too alkaline (pH>11.5), Cr(OH)₃ willdissolve again:Cr(OH)₃+OH⁻→Cr(OH)₄ ⁻

Because H⁺ ions are reduced at the strip surface, the concentration ofH⁺ ions will decrease near the strip surface. Consequently, aconcentration gradient will be established adjacent to the stripsurface. FIG. 1 shows the Nernst diffusion layer adjacent to theelectrode (c_(s): surface concentration [mol·m⁻³], c_(b): bulkconcentration [mol·m⁻³], δ: diffusion layer thickness [m], x: distancefrom electrode [m]).

The term single plating step intends to mean that the Cr—CrOx isdeposited from one electrolyte in one deposition step. The deposition ofa complex Cr(HCOO)(H₂O)₃(OH)₂(s) on the surface of the substrate isimmediately followed by the formation of Cr-metal, Cr-carbide and someremaining CrOx when the deposition takes place at a current densitywithin regime II. The higher the current density used in regime II, thehigher the amount of Cr-metal in the final deposit (see FIG. 7).Obviously one can choose to subsequently deposit one or more layers.When one deposits for example 2 layers, then each of these layers wouldbe deposited from one electrolyte in one deposition step.

In the well-known Nernst diffusion layer concept, one assumes that astagnant layer of thickness δ exists near the electrode surface. Outsidethis layer, convection maintains the concentration uniform at the bulkconcentration. Within this layer, mass transfer occurs only bydiffusion.

The diffusion flux J at the strip surface is given by Fick's first law:

$J = {\lbrack {{mol}\; m^{- 2}s^{- 1}} \rbrack = {{D( \frac{\partial c}{\partial x} )}_{x = 0} = {D( \frac{c_{b} - c_{s}}{\delta} )}}}$

where D is the diffusion coefficient [m²s⁻¹].

In scientific literature, expressions for the diffusion layer thicknesshave been derived for many practical cases, like a rotating disk(Levich), a rotating cylinder (Eisenberg), a flow in a channel(Pickett), and also a moving strip (Landau). According to an expressionderived by Landau the diffusion flux at the strip surface isproportional with the strip speed to the power 0.92: J≈v_(s) ^(0.92).This means that the diffusion layer thickness becomes thinner atincreasing strip speed.

For normal strip plating processes, e.g. plating of tin, nickel orcopper, this increase of the diffusion flux with increasing strip speedis very advantageous, because then a higher current density can beapplied and a higher deposition rate is obtained. In the plating processof these metals metal ions are discharged (reduced) to metal at thecathode by means of an electrical current and the reduced metal ions(i.e. metal atoms) are deposited onto the cathode (the metal strip).

But, in case of CrOx deposition, this increase of the diffusion fluxwith increasing strip speed is counterproductive, because the surface pHincrease, which is required to deposit Cr(OH)₃, is thwarted(counteracted) by the faster transport (replenishment) of H⁺ ions fromthe bulk of the electrolyte to the strip surface. Thus, at a higherstrip speed an increasingly higher current density is required todeposit the same amount of Cr(OH)₃. FIG. 2 shows that the deposition ofCr(OH)₃ via electrolysis of H⁺ leading to increase of surface pH atcathode (i.e. steel strip). Once CrOx (in the form of e.g. Cr(OH)₃) isdeposited, part of this deposit is reduced to metallic Cr.

FIG. 3 shows the current density as a function of the strip speedrequired for depositing 60 mg·m⁻² Cr as Cr(OH)₃. These data wereobtained from a Rotating Cylinder Electrode (RCE) study by equating masstransfer rate equations for an RCE and a Strip Plating Line (SPL).Clearly, an increasingly higher current density is required to depositthe same amount of Cr(OH)₃ at a higher strip speed.

Higher current densities not only demand more powerful (and expensive)rectifiers, but also imply a higher risk of unwanted side reactions atthe anode, like the oxidation of Cr(III) to Cr(VI). Moreover, when moreH₂(g) is formed at the strip surface, an exhaust system with a largercapacity is required to stay below the explosion limit of thehydrogen-air mixture. And also, there is the increased risk of damagingthe catalytic layer on the anode at higher current densities.

Also, when more H₂(g) is formed at the strip surface, the risk ofpinhole formation in the coating as a result of H₂-bubbles adhering tothe metal surface increases as well.

The invention is therefore based on the notion to increase the diffusionlayer thickness, which is counterintuitive as most electrodepositionreactions benefit from a thin diffusion layer.

The inventors found that the diffusion layer thickness can be increasedby increasing the kinematic viscosity of the electrolyte.

The invention will now be explained further by means of a non-limitativeembodiment.

In WO2013143928 an electrolyte was used for the Cr—CrOx depositioncomprising 120 g·l⁻¹ basic chromium sulphate, 250 g·l⁻¹ potassiumchloride, 15 g·l⁻¹ potassium bromide and 51 g·l⁻¹ potassium formate. ThepH was adjusted to values between 2.3 and 2.8 measured at 25° C. by theaddition of sulphuric acid. Further investigations showed that it ispreferable to replace the chlorides by sulphates to prevent Cl₂(g)formation. The present inventors discovered that bromide in a chloridebased electrolyte does not prevent the oxidation of Cr(III) to Cr(VI) atthe anode as is wrongfully claimed in U.S. Pat. Nos. 3,954,574,4,461,680, 4,804,446, 6,004,448 and EP0747510, but bromide reduceschlorine formation. So, when chlorides are replaced by sulphates,bromide can be safely removed from the electrolyte, because it serves nopurpose anymore. By using a suitable anode the oxidation of Cr(III) toCr(VI) at the anode in a sulphate based electrolyte can be prevented.The electrolyte then consists of an aqueous solution of a Cr(III) salt,preferably a Cr(III) sulphate, a conductivity enhancing salt in the formof potassium sulphate and potassium formate as a chelating agent andoptionally some sulphuric acid to obtain the desired pH at 25° C. Thissolution is taken as a benchmark against which the invention iscompared.

TABLE 1a Trivalent chromium electrolyte with K₂SO₄ molar mass c ccompound [g · mol⁻¹] CAS No. [g · l⁻¹] [M] CrOHSO₄ × Na2SO₄ × 307.11[10101-53-8] 120 0.385 nH₂O 16.7 wt-% Cr (n = 0) potassium sulphate174.26 [7778-80-5] 80 0.459 (K₂SO₄) potassium formate  84.12 [590-29-4]51.2 0.609 (CHKO₂)

The pH was adjusted to 2.9 at 25° C. by the addition of H₂SO₄.

TABLE 1b Trivalent chromium electrolyte with Na₂SO₄ molar mass c ccompound [g · mol⁻¹] CAS No. [g · l⁻¹] [M] CrOHSO₄ × Na₂SO₄ × 307.11[10101-53-8] 120 0.385 nH₂O 16.7 wt-% Cr (n = 0) sodium sulphate 142.04[7757-82-6] 250 1.760 (Na₂SO₄) sodium formate  68.01 [141-53-7] 41.40.609 (CHNaO₂)

The pH was adjusted to 2.9 at 25° C. by the addition of H₂SO₄. Clearly,the solubility of Na₂SO₄ (1.76 M) is much higher than the solubility ofK₂SO₄ (0.46 M). For the electrodeposition experiments titanium anodescomprising a catalytic coating of iridium oxide or a mixed metal oxideare chosen. Similar results can be obtained by using a hydrogen gasdiffusion anode. The rotational speed of the RCE was kept constant at 10s⁻¹ (Ω^(0.7)=5.0). The substrate was a 0.183 mm thick cold rolledblackplate material and the dimensions of the cylinder were 113.3 mm×ø73mm. The cylinders were cleaned and activated under the followingconditions prior to plating.

TABLE 2 Pretreatment of the substrate step 1 step 2 cleaning activationsolution composition 50 ml · l⁻¹ Chela 25 g · l⁻¹ H₂SO₄ Clean KC-25Htemperature (° C.) 60 25   current density (A · dm⁻²) +1.5 (anodic) 0(dip) Time (s) 60 1.5

An Anton Paar Model MCR 301 Rheometer was used for the viscositymeasurements. The kinematic viscosity ν (m²·s⁻¹) can be calculated bydividing the measured dynamic viscosity (kg·m⁻¹·s⁻¹) by the density(kg·m⁻³). The conductivity was measured with a Radiometer CDM 83conductivity meter.

The results of the viscosity and conductivity measurements at 50° C. areas follows.

TABLE 3 Viscosity and conductivity dynamic viscosity kinematic (cP) =(0.01 density viscosity conductivity g · cm⁻¹ · s⁻¹) (g · cm⁻³) (m² ·s⁻¹) (S · m⁻¹) 80 g · l⁻¹ 1.02 1.181 8.64E−07 13.5 K K₂SO₄ 100 g · l⁻¹1.43 1.175 1.22E−06 13.1 Na Na₂SO₄ 150 g · l⁻¹ 1.57 1.209 1.30E−06 14.5Na Na₂SO₄ 200 g · l⁻¹ 1.81 1.245 1.45E−06 15.6 Na Na₂SO₄ 250 g · l⁻¹2.43 1.284 1.89E−06 15.0 K Na₂SO₄

Despite the conductivity of a potassium solution being higher than thatof a sodium solution for the same concentration, the conductivity of 250g·l⁻¹ sodium sulphate is higher than that of 80 g·l⁻¹ potassiumsulphate.

The last column of the table indicates whether potassium formate (51.2g/l or 0.609 M) or sodium formate (41.4 g/l, or 0.609 M) was used ascomplexing agent. The difference in formate also explains why theelectrolyte with 250 g/l Na₂SO₄ has a lower conductivity than theelectrolyte with 200 g/l Na₂SO₄.

The diffusion flux for a RCE is proportional with ν^(−0.344) (Eisenberg,J. Electrochem. Soc., 101 (1954), 306)J=0.0642D ^(0.644)ν^(−0.344) r ^(0.4)(c _(b) −c _(s))ω^(0.7)

-   -   with ω=2πΩ

Inserting the measured kinematic viscosity values (the diffusioncoefficient D is divided out, because it is a ratio), it is expectedthat the diffusion flux (and also the current) for the Na₂SO₄electrolyte will be 24% smaller than for the K₂SO₄ electrolyte:

$\frac{J_{{Na}_{2}{SO}_{4}}}{J_{K_{2}{SO}_{4}}} = {( \frac{1.89 \times 10^{- 6}}{8.64 \times 10^{- 7}} )^{- 0.344} = 0.76}$

When the current becomes smaller, also the potential will becomesmaller, because the potential is directly proportional with the currentfor all ohmic resistances (according to Ohm's law: V=IR) in theelectrical circuit. Neglecting polarisation resistances at theelectrodes, the rectifier power is given by:P=VI=I²R

where R represents the sum of all resistances in the electrical circuit(electrolyte, bus bars, bus joints, anodes, conductor rolls, carbonbrushes, strip, etc.). So, the expected rectifier power saving will beabout 42% (0.76²=0.58).

For a strip plating line, the expected rectifier power saving will evenbe much larger (60%!), because the diffusion flux is proportional withν^(−0.59) (Landau, Electrochem. Society Proceedings, 101 (1995), 108):

J = 0.01D^(0.67)ν^(−0.59)L^(−0.08)(c_(b) − c_(s))v_(s)^(0.92)$\frac{P_{{Na}_{2}{SO}_{4}}}{P_{K_{2}{SO}_{4}}} = {( \frac{1.89 \times 10^{- 6}}{8.64 \times 10^{- 7}} )^{{- 0.59} \times 2} = 0.40}$

Moreover, the conductivity of the Na₂SO₄ electrolyte is 11% larger,entailing an additional rectifier power saving.

The deposition of Cr in mg·m⁻² versus i (A·dm⁻²) shows a threshold valuebefore Cr—CrOx deposition starts, a peak followed by a sudden, steepdecline ending in a plateau. Switching from a K₂SO₄ to a Na₂SO₄electrolyte shows that a much lower current density is required forCr—CrOx deposition. For depositing 100 mg·m⁻² Cr—CrOx only 21.2 A·dm⁻²is required instead of 34.6 A·dm⁻² (see the arrows in FIG. 4). Thedecrease is larger than anticipated on the basis of the ratio indiffusion fluxes (0.61 versus 0.76), which is probably caused by theapproximate character of the deposition mechanism.

XPS measurements show that there is no significant difference in thecomposition of the Cr—CrOx deposits produced from a Na₂SO₄ or K₂SO₄electrolyte. The degree of porosity decreased with higher kinematicviscosity electrolytes due to the lower current densities required andthe consequently reduced formation of H₂(g)-bubbles. The samples with acoating weight of about 100 mg·m⁻² Cr—CrOx were also analysed by meansof XPS (Table 4).

TABLE 4 Samples analysed by means of XPS. Cr—CrOx Cr Cr- Cr total MetalCrOx carbide XPS XPS XPS XPS sample Type i t [mg · [mg · [mg · [mg · #sulphate [A dm⁻²] [s] m⁻²] m⁻²] m⁻²] m⁻²] 31 Na₂SO₄ 21.2 1.0 112.3 826.0 23.4 75 K₂SO₄ 34.6 1.0 117.3 75 6.3 35.4

The remainder is some Cr₂(SO₄)₃ (0.8 and 0.6 mg·m⁻² respectively)

The current density for depositing 100 mg/m² Cr (which is a suitabletarget value for many applications) and the current density at which themaximum amount of Cr is deposited are given in Table 5. Theconcentration of the conductivity salt is limited by its solubilitylimit.

TABLE 5 Required current density for depositing 100 mg/m² Cr. concen-kinematic current density conductivity concentration tration viscosity100 mg m⁻² Cr salt (g l⁻¹) (M) (10⁻⁶ m² s⁻¹) (A dm⁻²) KCl 250 3.35 0.8734.5 K₂SO₄ 80 0.46 0.81 35.5 Na₂SO₄ 100 0.70 1.22 25.9 Na₂SO₄ 150 1.061.30 23.8 Na₂SO₄ 200 1.41 1.45 21.7 Na₂SO₄ 250 1.76 1.89 21.2

Clearly, the required current density for depositing 100 mg/m² Cr isshifted to a much lower value by using sodium sulphate as theconductivity salt (indicated by the arrow in the exploded view of FIG.6) instead potassium chloride or potassium sulphate.

Beside the lower current densities and the associated obvious advantagethere is also the reduced risk of formation of Cr(VI) (in case ofCr—CrOx) as a result of unwanted side reactions at the anode at lowercurrent densities, the lifetime of the catalytic iridium oxide coatingis extended, and the exhaust system for H₂(g) can be (much) smaller,because less H₂(g) is generated.

In an embodiment of the invention one or both sides of the electricallyconductive substrate moving through the line is coated with a Cr—CrOxcoating layer from a single electrolyte by using a plating process basedon a trivalent chromium electrolyte that comprises a trivalent chromiumcompound, a chelating agent and a conductivity enhancing salt, whereinthe electrolyte solution is preferably free of chloride ions and alsopreferably free of a buffering agent. A suitable buffering agent isboric acid, but this is a potentially hazardous chemical, so if possibleits use should be avoided. This relatively simple aqueous electrolytehas proven to be most effective in depositing Cr—CrOx. The absence ofchloride and the preferable absence of boric acid simplifies thechemistry, and also excludes the risk of the formation of chlorine gas,and makes the electrolyte more benign because of the absence of boricacid. This bath allows the deposition of Cr—CrOx in one step and from asingle electrolyte, rather than forming the Cr metal first in oneelectrolyte and then producing a CrOx coating on top in anotherelectrolyte. Consequently, chromium oxide is distributed throughout thechromium-chromium oxide coating obtained from a one-step depositionprocess, whereas in a two-step process the chromium oxide isconcentrated at the surface of the chromium-chromium oxide coating.

According to U.S. Pat. No. 6,004,448 two different electrolytes arerequired for the production of ECCS via trivalent Cr chemistry. Cr metalis deposited from a first electrolyte with a boric acid buffer andsubsequently Cr oxide is deposited from a second electrolyte without aboric acid buffer. According to this patent application in a continuoushigh speed line the problem arises that boric acid from the firstelectrolyte will be increasingly introduced in the second electrolytedue to drag-out from the vessel containing the first electrolyte intothe vessel containing the second electrolyte and as a result Cr metaldeposition increases and Cr oxide deposition decreases or is eventerminated. This problem is solved by adding a complexing agent to thesecond electrolyte that neutralizes the buffer that has been introduced.The present inventors discovered that for the production of ECCS viatrivalent Cr chemistry only one simple electrolyte without a buffer isrequired. Even though this simple electrolyte does not contain a bufferit was found by the present inventors that surprisingly also Cr metal isdeposited from this electrolyte due to partial reduction of Cr oxideinto Cr metal. This discovery simplifies the overall ECCS productionenormously, because an electrolyte with a buffer for depositing Cr metalis not required as is wrongfully assumed by U.S. Pat. No. 6,004,488, butonly one simple electrolyte without a buffer, which also solves theproblem of contamination of this electrolyte with a buffer.

In an embodiment of the invention the diffusion flux of H⁺-ions from thebulk of the electrolyte to the substrate/electrolyte interface isreduced by increasing the kinematic viscosity of the electrolyte and/orby moving the strip and the electrolyte through the plating line inconcurrent flow wherein the metal strip is transported through theplating line with a velocity (v1) of at least 100 m·s⁻¹ and wherein theelectrolyte is transported through the strip plating line with avelocity of v2 (m·s⁻¹). Both result in a thicker diffusion layer whichis beneficial for the Cr—CrOx deposition by counteracting the increaseof pH by reducing the diffusion flux of H⁺-ions from the bulk of theelectrolyte to the substrate/electrolyte interface.

In an embodiment of the invention the kinematic viscosity is increasedby using a suitable conductivity enhancing salt in such a concentrationso as to obtain an electrolyte with a kinematic viscosity of at least1·10⁻⁶ m²·s⁻¹ (1.0 cSt) when the kinematic viscosity is measured at 50°C. Note that this does not mean that the electrolyte is solely used at50° C. The temperature of 50° C. is intended here to provide a referencepoint for the measurement of the kinematic viscosity. In a preferableembodiment of the invention the kinematic viscosity of the electrolyteis at least 1.25·10⁻⁶ m²·s⁻¹ (1.25 cSt), more preferably at least1.50·10⁻⁶ m²·s⁻¹ (1.50 cSt) and even more preferably 1.75·10⁻⁶ m²·s⁻¹(1.75 cSt), all when measured at 50° C. Although physically there is nolimit to the upper boundary of the kinematic viscosity, as long as theelectrolyte stays liquid, each increase will lead to a more viscouselectrolyte, and at some stage the viscosity will start to causepractical problems with increased drag-out (a more viscous liquid willstick to the strip) and more stringent wiping actions. A suitable upperlimit for the kinematic viscosity is 1·10⁻⁵ m²·s⁻¹.

In an embodiment of the invention the kinematic viscosity is increasedby using sodium sulphate as the conductivity enhancing salt. By usingthis salt, which has a high solubility in water, the conductivity can beincreased to the same level as potassium sulphate, or even exceed that,and simultaneously produce a higher kinematic viscosity.

In an embodiment of the invention the kinematic viscosity is increasedby using a thickening agent. The kinematic viscosity can also beincreased by making the electrolyte more viscous by adding a thickeningagent.

The thickening agent can be inorganic, for example a pyrogenic silica,or organic, for example a polysaccharide. Examples of suitablepolysaccharide gelling or thickening agents are cellulose ethers such asmethyl cellulose, hydroxypropyl methyl cellulose, hydroxyethylcellulose, ethyl cellulose or sodium carboxymethyl cellulose, alginicacid or a salt thereof such as sodium alginate, gum arabic, gum karaya,agar, guar gum or hydroxypropyl guar gum, locust bean gum.Polysaccharides made by microbial fermentation can be used, for examplexanthan gum. Mixtures of polysaccharides can be used and may beadvantageous in giving a low shear viscosity which is temperaturestable. An alternative organic gelling agent is gelatin. Syntheticpolymeric gelling or thickening agents such as polymers of acrylamide oracrylic acid or salts thereof, e.g. polyacrylamide, partially hydrolysedpolyacrylamide or sodium polyacrylate, or polyvinyl alcohol canalternatively be used. Preferably the thickening agent is apolysaccharide.

In an embodiment of the invention the chelating agent is sodium formate.By using sodium formate rather than e.g. potassium formate the chemistryis further simplified. The composition of the deposited layers isunaffected by this change.

In another embodiment of the invention the thickness of the diffusionlayer is increased by moving the strip substrate and the electrolytethrough the strip plating line in concurrent flow wherein the ratio of(v1/v2) is at least 0.1 and/or at most 10. If v1/v2=1, then the stripsubstrate and the electrolyte move at the same speed. It is preferablethat the flow regime is a laminar flow. Turbulence will adversely affectthe thickness of the diffusion layer.

In an embodiment of the invention the ratio of (v1/v2) is at least 0.25and/or at most 4. In a preferable embodiment of the invention the ratioof (v1/v2) is at least 0.5 and/or at most 2.

In an embodiment of the invention a plurality (>1) of Cr—CrOx coatinglayers is deposited onto one or both sides of the electricallyconductive substrate, wherein each layer is deposited in a single stepin subsequent plating cells, in subsequent passes through the sameplating line or in subsequent passes through subsequent plating lines.

The mechanism of deposition of CrOx is driven by the increase of thesurface pH due to the reduction of H⁺ to H₂(g) at the strip surface (thecathode). This means that hydrogen bubbles form at the strip surface.The majority of these bubbles are dislodged during the plating process,but a minority may adhere to the substrate for a time sufficient tocause underplating at those spots potentially leading to a small degreeporosity of the metal and metal oxide layer (Cr—CrOx). The degree ofporosity of the coating layer is reduced by depositing a plurality (>1)of Cr—CrOx coating layers on top of each other on one or both sides ofthe electrically conductive substrate. For instance: Conventionally, alayer of chromium (Cr) is first deposited and then a CrOx layer isproduced on top in a second process step. In the process according tothe invention Cr and CrOx are formed simultaneously (i.e. in one step),indicated as a Cr—CrOx layer. However, even the product with a singlelayer, and thus having some porosity in the Cr—CrOx coating layer,passed all the performance tests for a packaging application where thesteel substrate with the Cr—CrOx coating layer is provided with apolymer coating. Its performance is thus comparable to the conventional(Cr(VI)-based!) ECCS material with a polymer coating. The degree ofporosity is reduced by depositing a plurality (>1) of Cr—CrOx coatinglayers on top of each other on one or on both sides of the electricallyconductive substrate. In this case each single Cr—CrOx layer isdeposited in a single step, and multiple single layers are depositede.g. in subsequent plating cells or in subsequent plating lines, or bygoing through a single cell or plating line more than once. This furtherreduces the porosity of the Cr—CrOx coating system as a whole.

In between the deposition of the multiple layers, it may be desirable,or even necessary, that the hydrogen bubbles are removed from thesurface of the strip. This may happen e.g. by the strip exiting andre-entering the electrolyte, by using a pulse plate rectifier or by amechanical action such as a shaking action or a brushing action.

In a preferable embodiment of the invention the electrolyte consists ofan aqueous solution of chromium (III) sulphate, sodium sulphate andsodium formate, unavoidable impurities and optionally sulphuric acid,the aqueous electrolyte having a pH at 25° C. of between 2.5 and 3.5,preferably at least 2.7 and/or at most 3.1. During plating some materialfrom the substrate may dissolve and end up in the electrolyte. Thiswould be considered an unavoidable impurity in the bath. Also, whenusing not 100% pure chemicals to produce or maintain the electrolytethere may be something in the bath which was not intended to be there.This would also be considered an unavoidable impurity in the bath. Anyunavoidable side reactions resulting in the presence of materials in theelectrolyte which were not there in the beginning are also considered anunavoidable impurity in the bath. The intention is that the bath is anaqueous solution to which only chromium (III) sulphate, sodium sulphateand sodium formate (all added in a suitable form), and optionallysulphuric acid to adjust the pH are added during the initial preparationof the bath and replenishment of the bath during its use. Theelectrolyte needs to be replenished during its use as a result of theoccurrence of drag-out (electrolyte sticking to the strip) and as aresult of the deposition of (Cr—)CrOx from the electrolyte.

Preferably the electrolyte for depositing the Cr—CrOx layer in a singlestep consists of an aqueous solution of chromium (III) sulphate, sodiumsulphate and sodium formate and optionally sulphuric acid, the aqueouselectrolyte having a pH at 25° C. of between 2.5 and 3.5, preferably atleast 2.7 and/or at most 3.1. Preferably the electrolyte containsbetween 80 and 200 g·l⁻¹ of chromium (III) sulphate, preferably between80 and 160 g·l⁻¹ of chromium (III) sulphate, between 80 and 320 g·l⁻¹sodium sulphate, more preferably between 100 and 320 g·l⁻¹ sodiumsulphate, even more preferably between 160 and 320 g·l⁻¹ sodium sulphateand between 30 and 80 g·l⁻¹ sodium formate.

Although the method according to the invention is applicable to anyelectrically conductive substrate, it is preferred to select theelectrically conductive substrate from:

-   -   tinplate, as deposited or flow-melted;    -   tinplate, diffusion annealed with an iron-tin alloy consisting        of at least 80% of FeSn (50 at. % iron and 50 at. % tin);    -   cold-rolled full-hard blackplate, single or double reduced;    -   cold-rolled and recrystallisation annealed blackplate;    -   cold-rolled and recovery annealed blackplate,

wherein the resulting coated steel substrate is intended for use inpackaging applications.

The second aspect of the invention relates to coated metal stripproduced in accordance with the method according to the invention.

The third aspect of the invention relates to a packaging produced fromthe coated metal strip produced in accordance with the method accordingto the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the concentration gradient of the H⁺-ions from at theelectrode (c_(s)) (the dashed block, at x=0) to the bulk concentration(c_(b)). The δ indicated the stagnant layer (diffusion layer thickness)in the Nernst diffusion layer concept. Outside this layer, convectionmaintains the concentration uniform at the bulk concentration. Withinthis layer, mass transfer occurs only by diffusion. The thickness of δis determined by the gradient of concentration at the electrode(∂c/∂x)_(x=0).

FIG. 2 is a schematical representation of the mechanism of thedeposition of Cr(OH)₃ on the substrate. Note that the H⁺-concentrationprofile is approximated by a straight line for simplicity. The δ againindicates the stagnant layer in the Nernst diffusion layer concept.

FIG. 3 shows how the required current density for the deposition of afixed amount of Cr(OH)₃ increases when the speed of the strip movingthrough a plating line increases. For electrodeposition based onMe^(n+)(aq)+n·e⁻→Me(s) the increase of current density would besufficient. For the mechanism based on deposition of Cr(OH)₃ the highspeeds result in a thinner diffusion layer thickness, and therefore theunwanted diffusion of H⁺ to the electrode speeds up as well.Measurements have indicated that for a line speed of 100 m·min⁻¹ acurrent density of 24.3 A·dm⁻² is needed for depositing 60 mg·m⁻²Cr—CrOx, whereas for 300 m/min 73 A·dm⁻² is needed and for 600 m·min⁻¹nearly 150 A·dm⁻² is needed.

FIG. 4 shows the Cr—CrOx vs. current density plots: a threshold valuebefore Cr—CrOx deposition starts, a peak followed by a sudden, steepdecline ending in a plateau.

FIG. 5 shows Cr—CrOx vs. current density plots for differentelectrolytes and for varying amounts of sodium phosphate.

FIG. 6 shows a cut-out from FIG. 5 which shows the current density fordepositing 100 mg/m² Cr, which is a suitable target value.

FIG. 7 plots the coating composition is vs. current density for 200 g/lNa₂SO₄ for a deposition time of 1 second, and in FIG. 8, the coatingcomposition weight is plotted vs. deposition time for a current densityof 20 A/dm² and for 200 g/l Na₂SO₄. Beyond the maximum current density(Regime III—as depicted in FIGS. 4 and 5, which for 200 g/l Na₂SO₄ isabout 25 A/dm²) the amount of Cr-metal drops and the coating isincreasingly composed of Cr-oxide with increasing current density. Inthe linear regime II towards the maximum the Cr-metal content increaseswith increasing electrolysis time mainly at the expense of Cr oxide. Theamount of Cr-carbide is about the same for all deposition times in FIG.8.

The invention claimed is:
 1. A method for producing a steel strip-coatedwith a mixed chromium metal-chromium oxide (Cr—CrOx) coating layer,comprising: transporting a moving steel strip through a plating linewith a velocity v1 of at least 100 m·min⁻¹, the plating line containinga single electrolyte solution comprising trivalent chromium and thesingle electrolyte solution moving with a velocity v2; wherein thesingle electrolyte solution is free of chloride ions, is free of abuffering agent and free of boric acid; wherein the single electrolytesolution consists of an aqueous solution of chromium (III) sulphate,sodium sulphate as a conductivity enhancing salt and sodium formate as achelating agent, unavoidable impurities and optionally sulphuric acid toadjust the pH, and optionally a thickening agent, forming the mixedchromium metal-chromium oxide (Cr—CrOx) coating layer upon one or bothsides of the steel strip, the Cr—CrOx coating layer being formed in aplating process from the single electrolyte solution in the platingline; wherein the steel strip acts as a cathode, reducing H⁺ to H₂(g) atan interface between the steel strip and the single electrolyte solutionto increase the pH at the interface between the steel strip and thesingle electrolyte solution and drive Cr—CrOx deposition for forming themixed chromium metal-chromium oxide (Cr—CrOx) coating layer; and usingthe sodium sulfate in a sufficient concentration for increasing akinematic viscosity of the single electrolyte solution to be 1·10⁻⁶m²·s⁻¹ (1.0 cSt) to 1·10⁻⁵ m²·s⁻¹ when measured at 50° C. and reducediffusion flux of H⁺ ions to the interface between the steel strip andthe single electrolyte solution, wherein the sodium sulphateconcentration is between 100 and 320 g·l⁻¹, wherein reducing thediffusion flux of H⁺ ions reduces current density at the interface todeposit Cr—CrOx.
 2. The method according to claim 1, wherein thekinematic viscosity is increased by using sodium sulphate as theconductivity enhancing salt at concentration from 150 to 250 g·l⁻¹. 3.The method according to claim 1, wherein the kinematic viscosity isincreased by using the conductivity enhancing salt in a concentrationfor the kinematic viscosity to be 1.75·10⁻⁶ m²·s⁻¹ (1.0 cSt) to 1·10⁻⁵m²·s⁻¹ when measured at 50° C.
 4. The method according to claim 1,wherein the single electrolyte solution has a pH at 25° C. of between2.5 and 3.5, wherein the kinematic viscosity is increased by usingsodium sulphate as the conductivity enhancing salt at concentration fromgreater than 100 to at most 250 g·l⁻¹ for the kinematic viscosity to be1.22·10⁻⁶ m²·s⁻¹ to 1.89·10⁻⁶ m²·s⁻¹ when measured at 50° C.
 5. Themethod according to claim 1, further comprising increasing the kinematicviscosity by using the thickening agent and chromium in the singleelectrolyte solution is solely from chromium (III) sulphate.
 6. Themethod according to claim 4, wherein the steel strip velocity v1 is 500to 900 m·min⁻¹, further comprising increasing the kinematic viscosity bytransporting the steel strip and the single electrolyte solution throughthe plating line in a concurrent flow arrangement, wherein the steelstrip and the single electrolyte solution move through the plating linein the concurrent flow arrangement at a ratio of v1/v2 of 0.25-4.
 7. Themethod according to claim 1, wherein the steel strip velocity v1 is 500to 900 m·min⁻¹, further comprising increasing the kinematic viscosity bytransporting the steel strip and the single electrolyte solution throughthe plating line in a concurrent flow arrangement, wherein the steelstrip and the single electrolyte solution move through the plating linein the concurrent flow arrangement at a ratio of v1/v2 of between 0.1and
 10. 8. The method according to claim 1, wherein a plurality (>1) ofCr—CrOx coating layers are deposited onto one or both sides of the steelstrip, wherein each layer is deposited in a single step in subsequentplating cells, in subsequent passes through the same plating line, or insubsequent passes through subsequent plating lines.
 9. The methodaccording to claim 1, wherein the single electrolyte solution has a pHat 25° C. of between 2.5 and 3.5, and wherein the unavoidable impuritiesare selected from the group consisting of dissolved steel strip, and animpurity in one or more chemicals included in the single electrolytesolution, the sodium sulfate at a concentration between 100 and 320g·l⁻¹.
 10. The method according to claim 1, wherein the steel strip,prior to being coated with the Cr—CrOx coating layer is one of:as-deposited tinplate, -or flow-melted tinplate; tinplate that isdiffusion annealed with an iron-tin alloy consisting of at least 80% ofFeSn (50 at. % iron and 50 at. % tin); single-reduced cold-rolledfull-hard blackplate, or double-reduced cold-rolled full-hardblackplate; cold-rolled and recrystallisation annealed blackplate;cold-rolled and recovery annealed blackplate; wherein the steel strip isintended for use in packaging applications after coating with theCr—CrOx coating.
 11. The method according to claim 1, wherein thethickening agent comprises a polysaccharide, wherein the kinematicviscosity is increased by using the polysaccharide and wherein thechromium in the single electrolyte solution is solely from chromium(III) sulphate.
 12. The method according to claim 1, wherein thesingle-electrolyte solution having a pH at 25° C. of between 2.5 and3.1, and wherein the unavoidable impurities are selected from the groupconsisting of dissolved steel strip, and an impurity in one or morechemicals included in the single electrolyte solution, wherein saidsodium sulfate is at a concentration between 100 and 320 g·l⁻¹.
 13. Themethod according to claim 1, the single electrolyte solution having a pHat 25° C. of between 2.7 and 3.1, and wherein the unavoidable impuritiesare selected from the group consisting of dissolved steel strip, and animpurity in one or more chemicals included in the single electrolytesolution, wherein said sodium sulfate is at a concentration from 150 toless than 320 g·l⁻¹.
 14. The method according to claim 1, wherein thesteel strip and the single electrolyte solution move through the platingline in a concurrent flow arrangement.
 15. The method according to claim1, wherein the steel strip and the single electrolyte solution movethrough the plating line in a concurrent flow arrangement at a ratio ofv1/v2 of between 0.1 and
 10. 16. The method according to claim 4,wherein the single electrolyte solution has a pH at 25° C. of between2.7 and 3.5.
 17. The method according to claim 16, wherein the singleelectrolyte solution has a pH at 25° C. of between 2.7 and 3.1, whereinthe kinematic viscosity is increased by using sodium sulphate as theconductivity enhancing salt at concentration from 150 to 250 g·l⁻¹ forthe kinematic viscosity to be 1.30·10⁻⁶ m²·s⁻¹ to 1.89·10⁻⁶ m²·s⁻¹ whenmeasured at 50° C. and wherein the sodium formate is at concentrationbetween 30 and 80 g·l⁻¹.